A diesel engine control apparatus including an SCR catalyst for purifying NOx in exhaust gas has a problem in that the catalyst does not reach the activating temperature immediately after activation of the engine or when low-load operation at a low exhaust temperature is continued, which leads to a low trend of the NOx purification state of the catalyst and insufficiency in the NOx purification.
Patent Documents 1 to 4 (identified below) disclose techniques for controlling an operation state of an engine in accordance with the temperature of a catalyst. The techniques control various engine control parameters such as an exhaust gas recirculation (EGR) rate, a fuel injection pressure, or a fuel injection timing in order to control the engine, and especially, to improve the fuel consumption when the temperature of the catalyst has become the activating temperature.    Patent Document 1: Japanese Unexamined Patent Application No. 2003-301737    Patent Document 2: Japanese Unexamined Patent Application No. 2006-37769    Patent Document 3: Japanese Unexamined Patent Application No. 2006-200473    Patent Document 4: Japanese Unexamined Patent Application No. 2011-241775